VII.
RADIO ASTRONOMY
Academic and Research Staff
Prof. A. H.
Prof. B. F.
Barrett
Burke
Prof. R. M. Price
J. W. Barrett
D. C.
C. A.
Papa
Zapata
Graduate Students
M. S. Ewing
W. W. Gebhart
L. P. A. Henckels
H. F. Hinteregger
P. C. Myers
G. D. Papadopoulos
P. W. Rosenkranz
P.
T.
T.
W.
R.
T.
L.
J.
Schwartz
Wilheit, Jr.
Wilson
Wilson
RESEARCH OBJECTIVES AND SUMMARY OF RESEARCH
1. Development of long-baseline interferometry techniques and studies of small
angular diameter sources. The use of highly stable atomic-frequency standards for
frequency and time control has made possible the construction of radio interferometers
of arbitrarily long baseline. Separate stations have been operated simultaneously at the
Haystack facility of Lincoln Laboratory, M. I. T., the National Radio Astronomy Observatory at Green Bank, West Virginia, and the Hat Creek Observatory of the University
of California at Berkeley, and the Onsala Observatory of Chalmers University, Sweden.
The maximum spacing used, thus far, has been 43, 000, 000 wavelengths at OH frequencies, which gives 0. 004 second of arc fringe spacing. A map of the OH emission region
in the radio source W3 has been constructed, and shows an assemblage of discrete
sources, the smallest of which has an angular size of 0. 0045 second of arc.
The
observations continue, both at OH and higher frequencies. Observations have been made
to check the validity of general relativity and to apply the technique to geophysical
problems.
2. Observations of interstellar OH. Single-antenna studies of the 18-cm lines of
OH have continued with the antennas of Lincoln Laboratory, M. I. T., or the National
Radio Astronomy Observatory, Green Bank, West Virginia, used. Strong OH emission
from infrared stars was discovered in July 1968, and a survey of such stars is now
under way.
Further studies of 018H continue in an attempt to shed light on maser the-
ories and the 016/018 interstellar abundance ratio.
3. Study of the microwave continuum from galactic and extragalactic radio sources
at short centimeter wavelengths. The variable quasi-stellar radio sources are observed
at 3. 75 cm and 2. 0 cm wavelengths to determine their time-variant spectra and polarization properties.
4. Study of microwave emission and absorption by the terrestrial atmosphere, with
particular emphasis on meteorologcial satellite application. This work has included
ground-based observations of atmospheric water vapor and ozone, and balloon observations of microwave emission by molecular oxygen. Up-looking balloon observations have
been conducted to determine the line absorption coefficient, and down-looking flights have
been carried out to test inversion techniques for recovering the temperature profile.
5. Aperture synthesis. A 2-cm interferometer is nearly complete and will be used
to make high-resolution studies of the bright radio sources. The interferometer will
This work was supported principally by the National Aeronautics and Space Administration (Grant NGL 22-009-016) and the National Science Foundation (Grant GP-8415),
and in part by the National Science Foundation (Grant GP-7156) and the Joint Services
Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract
DA 28-043-AMC-02536(E).
QPR No. 92
(VII.
RADIO ASTRONOMY)
make use of an on-line computer for real-time data reduction, and is intended to serve
as a prototype for further high-resolution instruments for continuum studies.
6. Pulsar antenna. An antenna is being constructed for use at low frequencies
(300 MHz and lower). The principal use ot the antenna will be to study pulsars at a
variety of frequencies, and to search for new pulsars. The antenna is equivalent in
aperture to a 200-ft dish.
A. H.
A.
Barrett, B. F. Burke
PULSAR FACILITY
The design and testing of a prototype 143-MHz feed for the 16-50 ft dishes is now
complete. Each feed consists of two orthogonal end-fire antennas. Each end-fire
antenna consists of 4 half-wave folded dipoles spaced by a quarter wavelength and with
a phase shift of 900 provided by the transmission line between the elements. The
resulting power reception pattern for each feed antenna has a front-to-back ratio of
approximately 20 dB and a 28 dB minimum toward the horizon to reduce reception of
The feed will provide approximately a cosine squared distribution
of field across the aperture of the 50 ft dishes. The illumination at the edge of the
local interference.
reflector is -10 dB.
Each column of 4
Open wire transmission lines to the dishes are being installed.
dishes in the North-South direction is connected to a central point via the equal length
transmission lines.
At the central point the 4 signals thus obtained can be combined
to give a multibeam response for the entire array.
A ten-channel receiver has been designed and is being built. It employs a 100-kHz
bandwidth for each channel and covers a 1-MHz band at 143 MHz, the selected observing
frequency.
B. F. Burke, R. M. Price, M. S. Ewing
B.
MILLIMETER-WAVE ABSORPTION BY
ATMOSPHERIC OXYGEN
The atmospheric oxygen program
- 5
has been continued during the past year. There
have been four flights of the previously described balloon-borne radiometer.2 See
Table VII-1 for general information on the flights. Two flights were made with the
antenna axes pointing upward at zenith angles of 600 and 750 to measure the absorption
line shape. Two other flights were made with the antennas pointing downward at nadir
angles of 00 and 600 in an attempt to obtain data that could be used to infer the atmospheric temperature profile. In all flights the local-oscillator frequency was 64.681 GHz,
corresponding to the 21+ oxygen resonance (64. 687 GHz). Three intermediate frequencies were used, 20, 60, and 275 MHz. Only the first of these flights has been analyzed
QPR No. 92
0
Table VII-1.
Flight
#
Date
L.O.
Frequency
General flight information.
Float
Altitude
Descent
Purpose
Comments
380-P
Feb 12 1968
64.681 GHz
30 km
valve
(slow)
To investigate
atmospheric
absorptions
near 21+ 0)
line
Good
flight
5 hours
381-P
Feb 24 1968
64.681 GHz
30 km
valve
(slow)
same
Good
flight
5 hours
406-P
Jun 27 1968
64.681 GHz
36 km
parachute To measure
(fast)
atmospheric
temperature
profile
below balloon
Interference
problem
on 20 mc
otherwise
good 9
hours
408-P
Jul 5 1968
64.681 GHz
35 km
parachute
(fast)
lost
local
oscillatox
after 2
hours
a,
same
_I
25P0
20.0
IS
C,
""
0.0
T
900
800
700
C
o1000
1200
1100
1300
TIME(CST)
Fig. VII-1.
Height vs time, Balloon Flight 380-P.
200-
-
200ISO-
0
I
2W--
10-
150-
z
IOD
z oo
1oo-
7o
ao
900
00
I
1200
1
1100
WO
1300
so0
Fig. VII- 2. Antenna temperature vs time
20-cm channel, 60* Antenna
Flight 380-P.
QPR No. 92
-
0oo
Io000
100
1200
1300
(CS)
nTIME
TIME
(CS1)
_ L
__
Fig. VII-3. Antenna temperature vs time
60-MHz channel, 60* Antenna
Flight 380-P.
--
0
-- -
250
250-
~200
150120No
z
100-
50
3-
-0
-
-50 700
m
m
1000
100
110]0
1200
1200
1300
00
1300
TIME(CSD)
0
1000
3
TIME(CM)
Fig. VII-4. Antenna temperature vs time
275-MHz channel, 60* Antenna
Flight 380-P.
Fig. VII-5. Antenna temperature vs time
20-MHz channel, 750 Antenna
Flight 380-P.
2W50
250-
200
S200R
150-
z
1o
z
50-
50
11
-0
-50
700
0M
900
1000
1100
1200
1300
00
TIME(CST)
Fig. VII-6. Antenna temperature vs time
60-MHz channel, 75* Antenna
Flight 380-P.
900
1000
1100
1200
100
TIME(CST)
Fig. VII-7. Antenna temperature vs time
275-MHz channel, 750 Antenna
Flight 380-P.
QPR No. 92
-
m
I
(VII.
RADIO ASTRONOMY)
enough to be worth discussing now.
Meeks and Lilley 6 compared the absorption data then available (1962)
Van Vleck-Weisskopf
7
theory and found that agreement required a pressure-broadened
linewidth, 6, not quite proportional to the pressure,
on altitude, h,
with the
P.
Rather, they found it depended
in the following way.
3 85
6 = ag(h) p(
g(h) = a
h -< H 1
g(h) = a + (1-a)(h-H 1 )(H 2 -H 1 )
H 1 > h > H2
g(h) = 1
h > H2
where T is the temperature in degrees Kelvin.
The values that Meeks and Lilley found to agree with the data are
a = 0.51
H
= 8 km
H 2 = 25 km
a = 1. 55 MHz/Torr.
Carter, Mitchell, and Reber
8
have done an extensive series of solar extinction measure-
ments between 53. 4 GHz and 56. 4 GHz from various altitudes 0 to 14. 75 km.
results similar to those of Meeks and Lilley.
They find
They find good fit with the values
a = 0.476
H1 = 8 km
H 2 = 25 km
a = 1. 86 MHz/Torr.
Their value for a
is considerably larger than the Meeks and Lilley value.
In the present experiment,
we measure the thermal radiation from the Earth's
atmosphere as a function of height from 0 to 30 km at frequencies approximately 20, 60,
and 275 MHz removed from the center frequency of the 21+ oxygen line (64. 687 GHz),
and at antenna zenith angles of 60' and 750.
QPR No. 92
The radiometer was continuously calibrated
(VII.
RADIO ASTRONOMY)
throughout the flight by cyclically switching the radiometer input in the order: 600 antenna,
hot calibration load, cold calibration load, 750 antenna, with a 4-min period. The effective temperatures of these loads differ from their physical temperatures because of
losses and reflections in the waveguides and the switches, and are, in general, different
for the two antennas. The radiometer was calibrated on the ground to determine these
effective temperatures, but the uncertainty in the cold-load temperature was still large.
During Flight 380-P some 5 hours of data was obtained.
the equipment, some of these data had to be rejected.
On account of malfunctions of
The data were edited for self-consistency and the points inconsistent with the bulk of
the data were rejected. The 20-MHz channel worked extremely well, and 93. 5% of its
data was retained. The 60-MHz channel was noisy, but 83. 8% of its data was useful.
The 275-MHz channel was extremely noisy and degenerated into complete nonsense after
approximately an hour and a half into the flight. Only the first 22. 1% of its data was kept.
Coincidentally with the flight, radiosondes were launched from the N. C. A. R. Balloon
Base in Palestine, Texas, and from 3 downrange sites, Shreveport, Louisiana, Lake
Charles, Louisiana, and Jackson, Mississippi. By using the atmospheric temperature
profiles thus obtained and the measured antenna patterns, it was possible to calculate,
on the IBM System 360, antenna temperatures as a function of height for various models.
Thus far, they have been calculated for a = 1 and a = 0. 51 (the Meeks and Lilley value),
for a range of values of a.
Since the cold-calibration temperatures were uncertain, they had to be determined
by comparing the data with each theory. The calibration temperatures were chosen to
minimize the ratio of the mean-square error to the mean-square signal noise. We shall
call the square root of this quantity the "normalized error." The best fit for the a = 1
model was obtained for a = 1.60 GHz/Torr and gave a normalized error of 1.91408
(12. 7°K). The a = 0. 51 (Meeks and Lilley) model gave a best fit of 1.853 (12. 2°K) for
a = 1. 65 MHz/Torr. For the Meeks and Lilley value of 1. 55 MHz/Torr we obtain a
normalized error of 1. 8577.
No calculations have been done for the a = 0.468 (Carter-
Mitchell-Reber) model, but it is not expected to be significantly different. For their
value of (1.86 MHz/Torr) and a = 0. 51, however, the relative error is 1. 8664.
The statistical error analysis for this experiment is not trivial and has not been
done yet. At any rate, it appears that the Meeks and Lilley model is substantially
correct.
The value of a may be slightly larger than theirs, but probably not as large
as the Carter-Mitchell-Reber value.
The experimental data, fitted to the a = 1. 65 MHz/Torr, a = 0. 51 theory, are shown
in Figs. VII-l through VII-7.
T. T. Wilheit, Jr., A. H. Barrett
QPR No. 92
(VII.
RADIO ASTRONOMY)
References
1.
2.
3.
4.
5.
6.
W. B. Lenoir and J. W. Kuiper, Quarterly Progress Report No. 75, Research Laboratory of Electronics, M. I. T., October 15, 1964, pp. 9-18.
W. B. Lenoir, Quarterly Progress Report No. 77, Research Laboratory of Electronics, M.I.T., April 15, 1965, pp. 20-33.
W. B. Lenoir, Quarterly Progress Report No. 79, Research Laboratory of Electronics, M.I.T., October 15, 1964, pp. 17-19.
W. B. Lenoir, Quarterly Progress Report No. 82, Research Laboratory of Electronics, M.I.T., July 15, 1966, pp. 36-41.
W. B. Lenoir, Quarterly Progress Report No. 84, Research Laboratory of Electronics, M.I.T., January 15, 1967, pp. 42-47.
M. L. Meeks and A. E. Lilley, J. Geophys. Res. 68, 1683-1703 (1963).
"The Absorption of Microwaves by Oxygen,"
Phys.
Rev.
71,
7.
J. H. Van Vleck,
413-424 (1947).
8.
C. J. Carter, R. L. Mitchell, and E. E. Reber, "Oxygen Absorption Measurements in the Lower Atmosphere," J. Geophys. Res. 73, 3113-3120 (1968).
QPR No. 92